An argumentative essay has three parts: introduction, body, and conclusion. The introduction and conclusion will normally consist of one paragraph each, while the length of the body will depend on the case you are making, and the word limit, if you have one.
In the introduction, state your thesis (the point you are attempting to prove), and say why it is important. The body is the part of the essay that is most difficult to structure. Plan the points you are going to make to support your case and work out the logical order in which to place them. Sometimes the principle of organization will be obvious. For instance, if you are arguing that a series of historical events led to a certain outcome, you will generally want to list the events in chronological order. However, the most important point is that there should be a principle of organization. Work out what your reader needs to know in order to be convinced of your case and decide what is the clearest and most convincing order in which to set out the information. In the absence of a chronological organizing principle, your strongest point should come at the end.
Generally, each point should be made in a separate paragraph. State the idea as clearly and succinctly as possible, then follow the statement with logical justification, data, and examples. Ideally, the conclusion should not merely summarize your case, but give some idea of the wider implications of what you have shown. What happens before and after your first draft is more important than the draft itself. Plan the essay carefully, listing all the points you are going to make in order. Once you have finished the draft, look at it critically as a reader, identifying and dealing with any flaws in the argument and rearranging (or cutting out) anything that looks out of place.